Recovery boiler optimization - Reduction rate control, case study

A recovery boiler’s main control variables of reduction rate were studied and the performance of the Valmet Recovery Analyzer’s online reduction rate measurement was evaluated in real recovery boiler conditions (at the Stora Enso Veitsiluoto mill in Kemi, Finland). The main objectives were to examine which control variable had the most significant effect on reduction rate and to find out how well the analyzer worked under changing burning conditions.

Valmet Recovery AnalyzerThe Valmet Recovery Analyzer (Alkali-R) is the third-generation Valmet process titrator (Kokkonen 2014) specially developed for online monitoring and closed loop control of the recovery boiler and causticizing process. The analyzer is a fully automatic, online sampling and titration analyzer for green liquor, causticizing and white liquor. Using a field-proven online sampling system and autotitrator, it provides outputs for control, based on standard process chemistry titration results.

A set of step change adjustments were performed on a recovery boiler’s main control variables in order to create desired changes in burning conditions. Evaluation of the analyzer performance was done by comparing measured reduction rate response to expected theoretical behavior, i.e. the reduction rate actual response to performed changes versus the reduction rate expected response according to theoretical knowledge.

The analyses of performed tests indicated that the reduction rate measurement provided reasonable and reliable results under changing burning conditions. Because of the complex nature of recovery boiler process and char bed chemistry, there are several process variables which affect reduction rate but cannot be manipulated, let alone kept constant.

A high reduction rate has a significant positive impact to the entire liquor recovery cycle. It supports maximized production and energy efficiency of the pulp mill, due to stronger white liquor concentration and higher black liquor heat value. An optimized recovery boiler with reduction rate control can now maintain high energy efficiency targets in variable process circumstances.

For more information on reduction rate control, view the Valmet white paper at THIS link, or talk to your Valmet representative.